The Ministry of Defence is preparing to launch an investigation into why the 'execution' of a wounded Taliban fighter only came to light when civilians stumbled across a video of the murder on a soldier's laptop.

The Royal Marine, known only as Marine A, was found guilty last week of murdering the Taliban insurgent in Helmand Province, Afghanistan, two years ago.

The 39-year-old is the first British soldier to be found guilty of a war crime in Afghanistan.

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Evidence: Footage captured by a camera mounted on the helmet of a Royal Marine and played during the court martial proceedings recorded the moment the soldier shot dead the insurgent in an Afghan field

The MoD has said no final decision has been taken on whether an inquiry will be launched but a senior marines source told The Sunday Times: 'We have been told to be prepared for an inquiry into the incident and what happened afterwards.'

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Marine A was convicted after footage emerged of him shooting the insurgent at close range in the chest.

As the man convulsed on the ground, Marine A told him: ‘There you are. Shuffle off this mortal coil, you ****. It’s nothing you wouldn’t do to us.’

Recording: The footage recorded on Marine B's helmet camera showed Marine A, a highly respected non-commissioned officer, order his men to move the captive out of sight of cameras on a surveillance balloon

He then turned to comrades and said: ‘Obviously this doesn’t go anywhere, fellas. I just broke the Geneva Convention.’

Two other soldiers, Marine B and Marine C, were cleared of murder. Footage was captured on a camera being worn on Marine B's helmet against orders.

Marine B tried to delete the footage but a clip was found by police in September 2012.

It is believed the clip may have been passed around like a war trophy.

A total of 30 British soldiers have been investigated over shooting incidents involving members of the Afghan Security Forces or insurgents since 2006.

A foreign field: After a two-and-a-half week trial, a seven-strong military board found Marine A guilty. His comrades named only as Marine B and Marine C were cleared after nine hours of deliberations

Of the 30 soldiers, marines and airmen investigated in 22 separate inquiries, only three have faced court martial and only one – Marine A – has been convicted

A Facebook page has been set up called Free Marine A which 10,000 people joined in less than a day.

But speaking during a visit to Helmand province, Defence Secretary Phillip Hammond said nobody on deployment in Afghanistan has called for clemency for the Royal Marine.

Mr Hammond said it was clear the murder was against the values of both Britain and the armed forces, in an interview with the Sky News Murnaghan programme.

The Defence Secretary's comments echoed the views of General Sir Nicholas Houghton, the Chief of the Defence Staff of the British Armed Forces, who said the Army must be 'immaculate' in upholding judicial process.

Audio: 'It's nothing you wouldn't do to us,' a soldier is heard saying in the recording after the shot is fired

Mr Hammond said: 'I have not heard any such suggestions (for clemency) here. People here understand part of what makes us different from the insurgents and the terrorists that we are going after is we maintain a certain standard.

'These are standards that are core to our values as a society and core to the values of the British Armed Forces. This is an isolated incident, I believe, one individual who has let the side down.

Phillip Hammond said it was clear the murder was against the values of both Britain and the armed forces

'It is not indicative of the kind of behaviour that people in the British Armed Forces condone or expect to indulge in.

'I have heard no suggestion since I have been here there is any request for special treatment for anyone convicted of the crime of murder.'

Speaking on the BBC One Andrew Marr Show, Gen Houghton said: 'My position on this is no serviceman or woman of the British Armed Forces is above the law - not above the law of the country, international law or the law of armed conflict.

'This was a heinous crime. Judicial process has found this individual guilty.

'It would be quite wrong for the armed forces to adopt some special pleading, some sort of exemption.

'If we try to put ourselves beyond the law or expect special provision from the law, then we start to erode the position where we have a moral ascendancy over those who are our enemies and that is the wrong thing to do.

'There is a due process that will lead to a sentencing. It's for that process to determine whether or not any form of clemency should be shown in the sentencing... we would not want our position to be eroded.

'Those in authority over the armed forces should not request any form of leniency... we should be immaculate in these respects.

'Murder is murder, this is a heinous crime, thankfully it is an exceptional act in terms of the conduct of our armed forces.'